1. Field of the Invention
This invention relates to a lithium secondary battery and, more particularly, to a lithium secondary battery having an improved negative electrode.
2. Discussion of the Related Art
In recent years, a nonaqueous electrolyte battery using lithium as a negative electrode active material has attracted attention as a high energy density battery. Of such nonaqueous electrolyte batteries, a primary battery using a light metal such as lithium, sodium, or aluminum as a negative electrode active material and manganese dioxide (MnO.sub.2), carbon fluoride [(CF).sub.n ], thionyl chloride (SOCl.sub.2), or the like as a positive electrode active material is already widely used as a power source of a timepiece or an electric calculator, or as a backup battery of a memory.
In addition, as the sizes and weights of various types of electronic equipment, such as communication equipment or VTR devices and so on, have been decreased, a demand for a secondary battery having a high energy density which can be suitably used as a power source of such equipment has been increased, and the nonaqueous electrolyte secondary battery has been actively studied. For example, a nonaqueous electrolyte secondary battery using lithium as a negative electrode and an electrolyte prepared by dissolving an electrolytic salt such as LiClO.sub.4, LiBF.sub.4, LiAsF.sub.6, or LiPF.sub.6 in a nonaqueous solvent such as propylene carbonate (PC), 1,2-dimethoxyethane (DME), .gamma.-butyrolactone (.gamma.-BL), or tetrahydrofuran (THF) has been studied. In addition, a compound which topochemically reacts with lithium such as TiS.sub.2, MoS.sub.2, V.sub.2 O.sub.5, or V.sub.6 O.sub.13 has been studied as a positive electrode active material.
The above secondary battery, however, has not been put into practical use yet. This is mainly because the charge/discharge efficiency of the battery is low and the number of charge/discharge times or cycle life is short. It is assumed that this is because the lithium negative electrode becomes degraded due to a reaction with the electrolyte. That is, lithium dissolved in an electrolyte as lithium ions upon discharge reacts with a solvent and the surface of the lithium is partially deactivated when it precipitates upon charging. Therefore, when charge/discharge is repeated, lithium is precipitated in the form of dendrites or small spheres, or is separated from the collector.
For these reasons, carbonaceous materials which are able to absorb or release lithium such as coke, sintered resin, carbon fiber or thermally decomposed epitaxial carbon, have been used to prevent the degradation of a negative electrode caused by reaction between lithium and nonaqueous electrolyte solution or by dendrite precipitation. However, because of the small absorbing-releasing capacity of lithium ion, the specific capacity of such a negative electrode is relatively small. Theoretically, increasing the absorbability of lithium ion should enlarge the charging capacity. However, such an increase has been difficult to achieve because the structure of the carbonaceous material deteriorates or the solvent in the electrolyte decomposes. Furthermore, there is a problem that when charging current density is elevated, the absorbed lithium quantity releases less metallic lithium. As a result, it is difficult to improve the cycle life of a lithium secondary battery including such a negative electrode.